The present invention is directed to diffractive optical structures and, more particularly, to diffractive optical structures which shape or split a beam of light and in which a uniform output is required. Diffractive optical structures producing off-axis beams according to the invention may be advantageously used for beam shaping and splitting.
As used herein, a xe2x80x9cbeam shaperxe2x80x9d is an optical element used to alter the shape or energy distribution within a beam of light. Thus, a beam shaper may alter magnification of a light beam, the footprint of the beam when projected on a surface, the energy distribution within a beam, or some combination thereof. An example of altering the energy distribution of a beam is transforming a Gaussian light distribution to a uniform light distribution. Beam shapers may be alternately and interchangeably referred to as xe2x80x9cbeam transformers.xe2x80x9d Also as used herein, xe2x80x9cbeam splitterxe2x80x9d refers to an optical element which divides a beam of light into two or more separate beams having similar characteristics.
FIG. 1 shows a conventional on-axis beam shaping assembly. An input beam 10, which has a Gaussian energy distribution, is transmitted by the diffractive beam shaper 11. The resultant shaped beam 12, which has a uniform energy distribution, strikes beam corrector 13 located a distance d from the beam shaper 11 along the optical axis of the input beam 10. The diffractive beam corrector 13 corrects a phase shift in the shaped beam 12 caused by the beam shaper 11. The beam shaping assembly shown is termed xe2x80x9con-axis,xe2x80x9d because the output beam 14 is located on the axis of the input beam 10. If the phase correction function was not desired, the assembly shown could consist of the beam shaper 11 alone.
Manufacturing tolerances can affect the output quality of beam shapers, such as that shown in FIG. 1, to a great degree. For example, for diffractive optics which are formed by dry etching, the etching processes are not exact, and the final optical shape may deviate slightly from the xe2x80x9cdesiredxe2x80x9d or xe2x80x9cperfectxe2x80x9d shape designed by an optical designer and sought to be etched. Such manufacturing errors or tolerances also occur with other methods of forming diffractive optics.
FIG. 2 shows the simulated output of an on-axis beam shaper, which is designed to produce a uniform beam, with various amounts of etch depth error. For the xe2x80x98perfectxe2x80x99 optic case 20 (i.e., where the designed shape is simulated with no fabrication or etch error), the peak to valley non-uniformity in the intensity of output beam is 2%. For the 0.5% etch depth error case 21, the peak to valley non-uniformity in the intensity of output beam increases to 10%. For the 3.0% etch depth error case 22, the peak to valley non-uniformity in the intensity of output beam increases to 46%. A typical etch depth tolerance to achieve a high yield in a conventional dry etching process is xc2x13.0%, which produces the 46% non-uniformity shown in plot 22. For many applications of beam transformers, such as lithography or holography, the desired uniformity of the beam is xc2x13.0%, which corresponds to a lower etch error than 3%, and hence cannot be attained with such a conventional high yield process.
It should be noted that the magnitude of the beam non-uniformity is a function of the magnification of the beam shaper. If the beam shaper produces a uniform beam that is much smaller than the input beam, for example one eighth, the additional non-uniformity caused by a 3.0% etch error can be as small as 2.0%. However, the effects of the 3% etching error quickly increase to 19% for a beam reduced to only one fourth size.
The non-uniformity observed in the output beams 21 and 22 in FIG. 2 is the result of the undesired orders produced by the diffractive interfering with the desired order of the output beam. Even though the energy in these orders may only be a few percent of the total input energy, they can have a profound affect on the uniformity of the beam, as illustrated in FIG. 2. The underlying problem is that all of the orders of an on-axis diffractive system are co-located symmetrically about the optical axis. Since a beam which is transformed in this manner is coherent, these co-located multiple order beams interfere and cause the non- uniformity shown in FIG. 2.
On-axis diffractive beam splitters, such as that shown in FIGS. 3a and 3b, suffer from similar problems of interference by undesired diffractive orders. Such an on-axis diffractive beam splitter may have an extremely tight tolerance for the etch depth of the diffractive, hence reducing the yield and making the cost of such a device impractical.
FIGS. 3a and 3b show perspective and side views, respectively, of an on-axis diffractive beam splitter that creates five beams. An input light beam 30 strikes a diffractive beam splitter 31, which is designed to split the input beam 30 into a 0th order beam 32 and four diffracted-order beams 33. The diffractive beam splitter shown is termed xe2x80x9con-axis,xe2x80x9d because the output beams 32 and 33 are located along a line which intersects the axis of the input beam 30. FIG. 3c shows the five beams in their one-dimensional, on-axis arrangement.
For the xe2x80x9cperfectxe2x80x9d optic case (not plotted), the peak to valley non-uniformity in the intensity of output beams 32 and 33 is 6% and the efficiency of the beam splitter is 92%. For the 3.0% etch depth error case, the peak-to-valley non-uniformity in the intensity of output beams is 26% and the efficiency is 91%. A typical etch depth tolerance to achieve a high yield in a conventional dry etching process is xc2x13.0%, which produces the 26% non-uniformity. This non-uniformity among split beams is caused by the co-location of the diffracted beams and the 0th order beam along a line. For many applications of beam splitters, such as communications and hole drilling or marking, the desired non-uniformity among the beams is less than xc2x15.0%, which corresponds to a lower etch error than 3%, and hence cannot be attained with such a conventional high yield process.
It is accordingly apparent that conventional on-axis diffractive beam shapers and splitters have extremely tight tolerances for the etch depth of the diffractive. Such tolerances lower the manufacturing yield, and thus make the cost of such devices impractical. Further, diffractive optics are wavelength sensitive, and the conventional on-axis configurations can only be used at the wavelength for which they are designed .
An object of the invention is to provide a diffractive optical element which substantially obviates one or more problems or limitations of conventional on-axis diffractive optical elements.
Another object of the invention is to design a diffractive beam splitter and/or diffractive beam shaper which is less sensitive to manufacturing errors and wavelength than conventional elements.
By designing a beam shaper or beam splitter that is off-axis by a defined minimum amount to separate the desired order(s) of the diffractive from the order(s) sensitive to manufacturing tolerances, the manufacturing difficulty of achieving the otherwise necessary tight tolerance in the etch depth needed for a very uniform beam may be eliminated. This off-axis configuration also allows a diffractive beam shaper or beam splitter to work over a large wave band.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided an off-axis beam shaper for producing an output beam of a desired order with a desired energy distribution, including an optical substrate; and a diffractive surface formed on the optical substrate to perform both a beam shaping function on an input beam and to spatially separate the output beam of the desired order from all other diffracted beams of different orders, thereby avoiding interference between the output beam and any other diffracted beam of a different order.
In another aspect, the invention includes an off-axis beam splitter for producing a plurality of substantially identical output beams, including an optical substrate; and a diffractive surface formed on the optical substrate to split an input beam into the plurality of substantially identical output beams and to translate the plurality of output beams away from an optical axis of the input beam.
Another aspect of the invention includes a method of shaping an input beam with diffractive optics, including diffracting an input beam to have a desired shape and energy distribution at a predetermined distance from the optic; andspatially separating an output beam having a desired order from other diffracted beams of different orders at the predetermined distance.
Still another aspect of the invention includes a method for producing a plurality of substantially identical and uniform output light beams, including splitting an input beam into the plurality of substantially identical output beams; and translating the plurality of output beams away from an optical axis of the input beam.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention.